Item #1 in the diagram.
The main part goes behind the housing with the spring tips clamping the lens & gasket to the housing. Installed by carefully pushing onto the lens/gasket/housing assembly from the ends. I've seen several lenses chipped where the clip tips were, and leaking into the housing.
The spring clips are plated but not very thick, practically biodegradable, even more so than the housing.
I've seen used housings with (usually rusty) clips on ebay, NOS lenses on ebay & mercado and reproduction gaskets on mercado.
